Although plant disease management policies are continuously developed all over the world, modern agriculture still faces devastating plant diseases. One of the most important diseases is potato late blight caused by the oomycete Phytophthora infestans that cause billion-dollar losses annually. The current control strategies of the disease are mainly fungicide application and breeding of cultivars with dominant resistance genes. The pathogen is notorious for its ability to overcome resistance and this is why many control strategies need always to be explored.
The prior art teaches a wide variety of materials that protect plants. Natural antimicrobial metabolites from plants having an effect on the growth of the phytopathogens have been used. Suppression of late blight development on tomato plants was achieved with ethanolic plant extracts of Paeonia suffruticosa and Hedera helix, both of which inhibited P. infestans zoospore release and germination (Röhner et al. 2004). Plant extracts of Rheum rhabarbarum and Solidago canadensis reduced the growth of P. infestans on potato leaves by directly inhibiting P. infestans growth (Stephan et al. 2005). Antibacterial, antifungal, and anti-oomycete activity of garlic juice has been reported and found to reduce the severity of late blight disease in potato plants (Slusarenko et al. 2008). A commercial product which shows an efficacy against P. infestans on potatoes is Elot-Vis®, which is based on an ethanolic plant extract (Stephan et al. 2005). Elot-Vis® showed effect against P. infestans on potato crops but only reduced disease if applied one day before P. infestans inoculation (Stephan et al. 2005). Elot-Vis® is directly toxic to the pathogen (Stephan et al. 2005).
Promising results achieved with the application of some plant defense inducing agents suggest that the principle of induced resistance might be employed in integrated disease management programs.
Resistance against P. infestans has been linked to the effect of a number of phenolic compounds like chlorogenic acid, p-coumaric acid, ferulic acid, rosmarinic acid, salicylic acid, 4-hydroxybenzaldehyde, and 4-hydroxybenzoate (Coquoz et al. 1995; Widmer and Laurent 2006). The commonly reported functions of phenolic compounds in the literature have been assigned to two main functions. Phenolic compounds can either have a direct toxic (antimicrobial) effect on the pathogens, e.g. soluble phenolic acids, or they can induce a barrier against pathogens by strengthening the cell wall to make it more resistant to the cell-wall degrading enzymes, e.g. cell wall-bound phenolics.